1. Field of the Invention
The present invention relates generally to a composite electrical insulator wherein a metal fitting is fixedly secured to a fiber-reinforced plastic rod at one end thereof.
2. Description of the Relater Art
A composite electrical insulator with such a constitution is known, e.g., from U.S. Pat. No. 4,654,478, wherein one end portion of the fiber-reinforced plastic rod is inserted into the bore in a sleeve portion of the metal fitting and the metal fitting is then fixedly secured to the plastic rod. To this end, the metal fitting is compressed radially inwardly onto the plastic rod so as to firmly clamp the rod. That is to say, by compressing the metal fitting radially inwardly, that region of the plastic rod situated opposite to the metal fitting is uniformly clamped to integrally connect the metal fitting with the plastic rod and prevent withdrawal of the plastic rod from the fitting even under a large tensile force.
In order to satisfy a severe requirement for a high tensile strength, the metal fitting is usually comprised of a high tension steel or ductile cast iron. However, due to the rigidity of the metal fitting which is considerably higher than that of the fiber-reinforced plastic rod, even a slight unevenness in the outer surface of the rod end portion or the inner surface of the bore in the metal fitting may cause a local deformation in adjacent outer surface region of the rod, thereby giving rise to considerable residual internal stresses. In this instance, when the insulator is applied with an external force, typically an axial tensile force, the internal stress is multiplied in the end portion of the rod which is clamped within the sleeve, causing a high degree of stress concentration and thereby giving rise to damage or breakage of the rod within a relatively short period. It has thus been considered necessary to perform highly accurate and precise machining with respect to the inner surface of the bore in the metal fitting and the outer surface of the rod end portion. Needless to say, such machining often makes it difficult to improve the manufacturing productivity and reduce the cost of the insulators.
Similar problems may arise also when the radially inwardly directed compression of the fiber-reinforced plastic rod exhibits a non-uniform distribution in the circumferential direction of the rod in any cross-section of the metal fitting. Therefore, it has been considered an indispensable condition for the insulators to have a structure wherein the plastic rod is compressed radially inwards with a practically satisfactory uniformity.